Fast detection of the presence of a target microbe in a liquid sample

ABSTRACT

A system is provided that includes a tray having a number of compartments for holding liquid samples and partitioning the liquid samples from one another. The liquid samples are prepared by adding an indicator configured to produce a characteristic change in light from the liquid samples when a target microbe metabolizes the indicator while the liquid samples are incubated. The system also includes a light sensor for sensing light from the liquid samples held in the tray while the plurality of liquid samples is incubated. The system further includes a processor coupled with the light sensor and configured to analyze the light from the liquid samples while the liquid samples are incubated to detect the characteristic change in light from one or more of the liquid samples if the target microbe is present in the liquid samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application Ser. No. 61/587,252, filed Jan. 17, 2012,and titled “FAST DETECTION OF THE PRESENCE OF A TARGET MICROBE IN ALIQUID SAMPLE,” which is herein incorporated by reference in itsentirety.

BACKGROUND

Escherichia coli, (commonly abbreviated E. coli) is a Gram-negative,rod-shaped bacterium that is commonly found in the lower intestine ofwarm-blooded organisms. Most E. coli strains are harmless, but someserotypes can cause food poisoning in humans, and are responsible forproduct recalls. For E. coli and related bacteria, fecal-oraltransmission is the major route through which pathogenic strains of thebacterium cause disease. Cells are able to survive outside the body fora limited amount of time, which makes them ideal indicator organisms totest environmental samples for fecal contamination.

SUMMARY

A system for fast detection of the presence of a target microbe in aliquid sample is disclosed. In one or more implementations, the systemincludes a tray having a number of compartments for holding liquidsamples and partitioning the liquid samples from one another. The liquidsamples are prepared by adding an indicator configured to produce acharacteristic change in light from the liquid samples when a targetmicrobe metabolizes the indicator while the liquid samples areincubated. The system also includes a light sensor for sensing lightfrom the liquid samples held in the tray while the plurality of liquidsamples is incubated. The system further includes a processor coupledwith the light sensor and configured to analyze the light from theliquid samples while the liquid samples are incubated to detect thecharacteristic change in light from one or more of the liquid samples ifthe target microbe is present in the liquid samples.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

DRAWINGS

The Detailed Description is described with reference to the accompanyingfigures.

FIG. 1 is a schematic illustrating a system for automatically detectingthe presence and/or absence of a target microbe in a liquid sample inaccordance with example implementations of the present disclosure.

FIG. 2 is a cross-sectional side elevation view illustrating a tray forholding a number of individual liquid samples, along with a divider forportioning a bulk liquid sample into the individual liquid samples inaccordance with example implementations of the present disclosure.

FIG. 3A is a schematic illustrating a tray for holding a number ofindividual liquid samples, along with a light source and a light sensorin accordance with example implementations of the present disclosure.

FIG. 3B is a schematic illustrating a tray for holding a number ofindividual liquid samples, along with a positionable light source and apositionable light sensor and light guide in accordance with exampleimplementations of the present disclosure.

FIG. 4 is a flow diagram illustrating a method for automaticallydetecting the presence and/or absence of a target microbe in a liquidsample in accordance with example implementations of the presentdisclosure.

DETAILED DESCRIPTION

Overview

Strains of E. coli are a major cause of foodborne illness. Generally, inorder to detect E. coli and other microbes, samples must be taken from asuspected source and incubated over a period of time (e.g., eighteen(18) hours) in order to determine whether the suspected source iscontaminated. After the incubation period, a tester must then manuallyexamine each sample and record the results of each test. In someinstances, this involves counting the number of individual samples thattest positive and then comparing this result to a pre-specified number(e.g., when a concentration of biological contaminants must be measured,rather than the presence of absence of one or more of the contaminants).The long waiting time and the potential for error involved in countingand logging numbers of samples can lead to critical delays in theidentification of potential sources of contamination. This may result infurther contamination of a population and/or a wider spread ofpotentially contaminated goods.

Accordingly, techniques are described for fast detection of the presence(and/or absence) of a target microbe, such as E. coli, in a liquidsample. The techniques employ an automated system that includes a trayhaving a number of compartments for holding liquid samples andpartitioning the liquid samples from one another. The liquid samples areprepared by adding an indicator configured to produce a characteristicchange in light from the liquid samples when a target microbemetabolizes the indicator while the liquid samples are incubated. Thesystem also includes a light sensor for sensing light from the liquidsamples held in the tray while the plurality of liquid samples isincubated. The system further includes a processor coupled with thelight sensor and configured to analyze the light from the liquid sampleswhile the liquid samples are incubated to detect the characteristicchange in light from one or more of the liquid samples if the targetmicrobe is present in the liquid samples.

Example Implementations

FIGS. 1 through 3 illustrate example systems 100 for detecting thepresence and/or absence of a target microbe in a liquid sample inaccordance with example implementations of the present disclosure.

It should be noted that for the purposes of the present disclosure,language such as “presence” and “absence,” when used with respect to thedetection (or lack thereof) of a target microbe, is not intended to beconstrued as limited to the detection of any quantity of the targetmicrobe. Rather, the terms “presence” and “absence” are used to refer toboth detection of one or more target microbes, as well as toquantification of microbial concentration of a target microbe within asample (e.g., with respect to a pre-specified concentration). In someimplementations, the presence of a target microbe within a sample mayrefer to the detection of any number of target microbes within thesample (e.g., when an example system 100 is used to detect a targetmicrobe within a sample of potable water). Correspondingly, the absenceof a target microbe within a sample may refer to the lack of detectionof any number of target microbes within the sample. In otherimplementations, the presence of a target microbe within a sample mayrefer to the detection of a microbial concentration greater than apre-specified concentration (e.g., when an example system 100 is used todetect a target microbe within a sample of waste water, incoming waterfor a water purification system, surface water, food testing, and soforth). Correspondingly, the absence of a target microbe within a samplemay refer to the detection of a microbial concentration less than apre-specified concentration. Further, a pre-specified concentration forestablishing the presence and/or absence of a target microbe may befurnished by a user (e.g., via a user interface) and/or may bedetermined according to guidelines published by a regulatory agency(e.g., the Environmental Protection Agency (EPA), the Food and DrugAdministration (FDA), and so forth).

An example system 100 includes a tray 102 for holding one or more liquidsamples 104. The tray has a number of compartments 106 for holding andpartitioning the liquid samples 104, keeping the liquid samples 104separate from one another in the tray 102. For example, in oneparticular configuration, the tray 102 may comprise between about fiftyand one hundred (50-100) compartments 106. However, this range isprovided by way of example only and is not meant to be restrictive ofthe present disclosure. Thus, the tray 102 may comprise more than onehundred (100) or fewer than fifty (50) compartments 106. Thecompartments 106 can have various sizes/volumes. For example, in someinstances, the compartments 106 may range in volume from about two onehundredths of a milliliter to about two milliliters (0.02 ml-2 ml).However, this range is provided by way of example only and is not meantto be restrictive of the present disclosure. Thus, the compartments 106may have other various volumes. Further, all of the compartments 106 mayhave uniform (e.g., identical) volumes. However, it is contemplated thatone set of compartments 106 (e.g., a group of twenty compartments) mayeach comprise one volume (e.g., two one hundredths of a milliliter (0.02ml)), and another set of compartments 106 (e.g., another group of twentycompartments) may each comprise another volume (e.g., two milliliters (2ml)). Further, in some implementations, the tray 102 can be at leastsubstantially transparent, allowing light to pass from the liquidsamples 104 through the material of the tray 102.

The liquid samples 104 are prepared by adding an indicator that producesa characteristic change in light (e.g., a change in light color in thevisible spectrum, ultraviolet (UV) spectrum, infrared (IR) spectrum, andso forth) from the liquid samples 104 when a target microbe (e.g.,Escherichia coli (E. coli)) is present in one or more of the liquidsamples 104. In implementations, the characteristic change in light maybe generated based upon the presence of discrete biological materialwithin a sample that indicates the presence and/or absence of the targetmicrobe, including, but not necessarily limited to: bacteria, fungi,living organisms, aggregates of proteins (e.g., enzymes, co-factors),and so forth. For example, a characteristic change in light may beproduced when the target microbe metabolizes the indicator (e.g., whilethe liquid samples 104 are incubated). The liquid samples 104 can beincubated in, for instance, an incubator assembly 108.

In implementations, the indicator is a testing medium comprising achemical and/or microbiological reactant. For instance, the indicatormay comprise a reagent that fluoresces when exposed to ultraviolet (UV)light in the presence of E. coli. Further, the indicator can be apreferred or primary nutrient for a target microbe that cannot besubstantially metabolized by other viable microbes that may be presentin the sample. A growth accelerant can also be added to the liquidsamples 104 to accelerate growth of the target microbe. The growthaccelerant can be configured to boost the growth of the target microbes(and possibly other viable microbes) through a lag growth phase andtoward a log growth phase. In implementations, other selective chemicalscan also be provided, including antimetabolites, antibiotics, and soforth.

The system 100 may include a divider 110 for receiving a bulk liquidsample and portioning the sample into individual liquid samples 104. Thebulk liquid sample can be prepared using one or more indicators, growthaccelerants, and so forth (e.g., as previously described). In suchimplementations, the compartments 106 of the tray 102 can be configuredto receive the liquid samples 104 from the divider 110. For instance,the divider 110 may comprise a second tray having side walls and asubstantially porous, flat horizontal surface there between, where thesurface is configured such that liquid introduced to the tray willevenly disperse across the flat surface and permeate through the poresto the compartments 106 of the tray 102. In some implementations, thetray 102 can be provided within a sealed package. This may reduce thepossibility of exposing a tester to the liquid samples 104. For example,a liquid sample 104 can be poured into the divider 110 and dispersedinto the compartments 106 of the tray 102. The tray 102 may then beplaced within the package, and the package sealed. It is contemplatedthat the package may be fabricated of a material that is at leastsubstantially transparent for allowing light to pass from the liquidsamples 104 through the packaging material.

In implementations, the divider 110 can include an array of pores and/orchannels 112, where, for example, one or more channels 112 correspond toeach compartment of the tray 102. When a bulk liquid sample is pouredinto the tray, the liquid sample is portioned into the liquid samples104 and flows through the channels 112 into the compartments 106 of thetray 102. It should be noted that the pores/channels 112 may be sizedand/or shaped differently to accommodate differently sized compartments106 of the tray 102. For example, smaller pores/channels 112 can be usedto provide less liquid sample 104 material to smaller compartments 106,while comparatively larger pores/channels 112 can be used to providemore liquid sample 104 material to larger compartments 106. Further, insome implementations, channels 112 may be formed using one or morestructures extending from the divider body. For example, channels 112may be formed in tubes (e.g., pipettes) extending from the body of thedivider 110. In some instances, pipettes may be configured to extendfrom the divider 110 some distance into one or more of the compartments106 (e.g., extending just past the opening of a compartment, extendinginto a compartment 106 proximate to the bottom of the compartment, andso forth). In implementations, the divider 110 may comprise anautosampler assembly used to deliver one or more aliquots of a liquidsample 104 to each compartment 106 of the tray 102.

The system 100 also includes a light sensor 116 for sensing one or morecharacteristics of light from the liquid samples 104 held in the tray102. In implementations, the light sensor 116 may be used toautomatically sense light from the liquid samples 104 while the liquidsamples 104 are incubated to provide real-time monitoring of the liquidsamples 104 (e.g., during incubation of the liquid samples). The lightsensor 116 may be configured as a photosensor/photodetector. Forexample, as shown in FIG. 3A, the light sensor 116 may comprise one ormore photosensor diodes, phototransistors, and so forth. Inimplementations, the light sensor 116 is capable of detecting light andproviding a signal in response thereto. Thus, the light sensor 116 mayprovide a signal by converting light into current and/or voltage basedupon the intensity of the detected light. For example, when a lightsensor 116 configured as a photodetector is exposed to light, multiplefree electrons may be generated, creating a signal comprised ofelectrical current. The signal may correspond to one or morecharacteristics of the detected light, such as intensity (e.g.,irradiance, etc.) of light incident upon the photodetector.

The light sensor 116 may comprise a Charge-Coupled Device (CCD), such asan image sensor sensitive to ultraviolet and/or visible light, an imagecapture device, such as a camera, and so forth. In implementations, thelight sensor 116 may be configured to sense light from only a portion ofthe liquid samples 104 at a given instant in time. For instance, thelight sensor 116 can be configured to sense light from the liquidsamples 104 held in the compartments 106 one at a time. As shown in FIG.3B, this can be implemented by a light sensor 116 configured to movefrom one compartment 106 or group of compartments 106 to another at atime, and/or by masking a group of compartments 106 from the lightsensor 116. Further, separate portions of an image (e.g., regions ofpixels from an image capture device) can be analyzed to determine lightemitted from separate ones of the compartments 106. In implementations,the light sensor 116 can be used with a light guide configured to directlight from one or more of the compartments to the light sensor 116 (orto a specific detection region thereof). The light guide can beimplemented as, for instance, an array of fiber optic cables, and soforth.

The system 100 may further include a light source 118 (e.g., one or morelight emitting diodes for emitting visible light, UV light, and/orinfrared light, as shown in FIG. 3A, a lamp for emitting visible light,UV light, and/or infrared light, as shown in FIG. 3B, combinationsthereof, and so forth) for illuminating the liquid samples 104 held inthe compartments 106 of the tray 102. In implementations, the lightsource 118 may be configured to illuminate only a portion of the liquidsamples 104 at a given instant in time. For instance, the light source118 can be configured to illuminate the liquid samples 104 held in thecompartments 106 one at a time. This can be implemented by a lightsource 106 that includes a plurality of LEDs which may be switched onand off, as shown in FIG. 3A, or by a light source 118 configured tomove from one compartment 106 or group of compartments 106 to another ata time, as shown in FIG. 3B, and/or by masking a group of compartments106 from the light source 118. The light source 118 can be used toprovide sufficient light for the light sensor 116 to measure acharacteristic change in light from the liquid samples 104. Forinstance, a UV light source 118 may be used to cause a reagent tofluoresce when exposed to ultraviolet (UV) light in the presence of atarget microbe such as E. coli.

The system 100 further includes a processor 124 coupled with the lightsensor 116. The processor 124 is configured to analyze one or morecharacteristics of the light from the liquid samples 104 as sensed bythe light sensor 116 to detect the characteristic change in light fromthe liquid samples 104 if the target microbe is present. For instance,the system 100 may comprise an electronic device/controller 122, whichmay include the processor 124, memory 126, and so forth. The processor124 provides processing functionality for the electronic device 122 andmay include any number of processors, micro-controllers, or otherprocessing systems and resident or external memory for storing data andother information accessed or generated by the electronic device 122.The processor 124 may execute one or more software programs whichimplement the techniques and modules described herein. The processor 124is not limited by the materials from which it is formed or theprocessing mechanisms employed therein and, as such, may be implementedvia semiconductor(s) and/or transistors (e.g., electronic IntegratedCircuits (ICs)), and so forth.

The memory 126 is an example of device-readable storage media thatprovides storage functionality to store various data associated with theoperation of the electronic device 122, such as the software program andcode segments mentioned above, or other data to instruct the processor124 and other elements of the electronic device 122 to perform thetechniques described herein. Although a single memory 126 is shown, awide variety of types and combinations of memory may be employed. Thememory 126 may be integral with the processor 124, stand-alone memory,or a combination of both. The memory 126 may include, for example,removable and non-removable memory elements such as Random Access Memory(RAM), Read Only Memory (ROM), Flash memory (e.g., a Secure Digital (SD)card, a mini-SD card, a micro-SD card), magnetic memory, optical memory,Universal Serial Bus (USB) memory devices, and so forth. In embodimentsof the electronic device 122, the memory 126 may include removableIntegrated Circuit Card (ICC) memory, such as memory provided bySubscriber Identity Module (SIM) cards, Universal Subscriber IdentityModule (USIM) cards, Universal Integrated Circuit Cards (UICC), and soon.

In implementations, the processor 124 may be used to automaticallyanalyze characteristics of the light from the liquid samples 104 whilethe liquid samples 104 are incubated to provide real-time analysis ofthe liquid samples 104 for detecting the presence and/or the absence ofthe target microbe. For example, the processor 124 can be configured toautomatically identify the presence of the target microbe in the liquidsamples 104 as soon as the characteristic change in light is detectedfor a pre-specified number of the liquid samples 104, which maycorrespond to, for instance, a concentration of the target microbewithin the liquid samples 104. The pre-specified number of the liquidsamples 104 can be stored in the memory 126. This may allow the presenceand/or the absence of the target microbe to be determined more quickly(e.g., without having to wait for the entirety of the incubationperiod).

In implementations, the most probable number method can be used todetermine a pre-specified number of liquid samples 104 in which acharacteristic change in light is needed to identify the presence and/orabsence of the target microbe. For example, a pre-specified number ofliquid samples 104 can be determined using one or more techniquesdescribed in Recles et al., “Most Probable Number Techniques” publishedin “Compendium of Methods for the Microbiological Examination of Foods”,3rd ed. 1992, at pages 105-199, and/or in Greenberg et al., “StandardMethods For the Examination of Water and Wastewater” (8th ed. 1992).

Statistical techniques can be used to project whether a particular groupof liquid samples 104 is likely to test positive (or negative) for thepresence and/or the absence of a particular target microbe within anincubation period for which the pre-specified number of liquid samples104 has been determined (e.g., using techniques referred to above). Forexample, if a characteristic change in light is identified for a certainnumber of liquid samples 104 less than the pre-specified number beforethe end of an incubation period, but within an intermediate timeinterval, the liquid samples 104 may be identified as containing aconcentration of target microbes necessary to establish their presencewithin the liquid samples 104. Further, the likely presence of thesample microbe may be returned as an intermediate result, alerting atester that the presence of the target microbe is likely indicated forthe liquid samples 104. Correspondingly, if a characteristic change inlight is not identified for a certain number of liquid samples 104 lessthan the pre-specified number before the end of an incubation period,but within an intermediate time interval, the liquid samples 104 may beidentified as containing a concentration of target microbes insufficientto establish their presence within the liquid samples 104. Further, thelikely absence of the sample microbe may be returned as an intermediateresult, alerting a tester that the absence of the target microbe islikely indicated for the liquid samples 104.

Example Process

Referring now to FIG. 4, example techniques for detecting the presenceand/or absence of a target microbe in a liquid sample are described.

FIG. 4 depicts a process 400, in an example implementation, fordetecting the presence and/or absence of a target microbe in a liquidsample using, for example, the system 100 illustrated in FIGS. 1 through3 and described above. In the process 400 illustrated, one or moreliquid samples are held, where the liquid samples have been prepared byadding an indicator configured to produce a characteristic change inlight from the plurality of liquid samples when a target microbemetabolizes the indicator while the plurality of liquid samples isincubated (Block 410). For example, with reference to FIGS. 1 through 3,liquid samples 104 can be prepared by adding an indicator that producesa characteristic change in light (e.g., a change in light color in thevisible spectrum, ultraviolet (UV) spectrum, infrared (IR) spectrum, andso forth) from the liquid samples 104 when a target microbe (e.g.,Escherichia coli (E. coli)) is present in one or more of the liquidsamples 104. The liquid samples 104 can be held in tray 102 havingcompartments 106 for partitioning the liquid samples 104, keeping theliquid samples 104 separate from one another in the tray 102. The tray102 can be held in incubator assembly 108 while the liquid samples 104are incubated.

Light can be automatically sensed from the liquid samples while theliquid samples are incubated (Block 420). For instance, with continuingreference to FIGS. 1 through 3, light sensor 116 can be used to senseone or more characteristics of light from the liquid samples 104 held inthe tray 102. The light sensor 116 can be used to automatically senselight from the liquid samples 104 while the liquid samples 104 areincubated to provide real-time monitoring of the liquid samples 104. Thelight sensor 116 may be configured as a photosensor/photodetector, animage capture device, and so forth.

One or more characteristics of the light from the liquid samples can beautomatically analyzed while the liquid samples are incubated to detectthe characteristic change in light from one or more of the liquidsamples if the target microbe is present (Block 430). For example, withcontinuing reference to FIGS. 1 through 3, system 100 may furtherinclude processor 124 coupled with the light sensor 116. The processor124 can be used to automatically analyze characteristics of the lightfrom the liquid samples 104 while the liquid samples 104 are incubatedto provide real-time analysis of the liquid samples 104 for detectingthe presence and/or the absence of the target microbe. Thus, in someimplementations, the presence of the target microbe in the liquidsamples can be determined as soon as the characteristic change in lightis detected for a pre-specified number of the liquid samples (Block432). For example, the processor 124 can be configured to automaticallyidentify the presence of the target microbe in the liquid samples 104 assoon as the characteristic change in light is detected for apre-specified number of the liquid samples 104, which may correspond to,for instance, a concentration of the target microbe within the liquidsamples 104.

CONCLUSION

Although the subject matter has been described in language specific tostructural features and/or process operations, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A system for detecting the presence of a targetmicrobe comprising: a tray configured to be received in an incubator forholding liquid samples during incubation, each of the liquid samplesincluding an indicator configured to produce a characteristic change inlight from the liquid sample when a target microbe metabolizes theindicator during incubation of the liquid sample, the tray including aplurality of compartments configured to partition the liquid samples onefrom another; a light sensor for individually sensing light from each ofthe liquid samples held separately in the plurality of compartments ofthe tray during incubation of the liquid samples, the light sensorconfigured to detect a characteristic of the light sensed from each ofthe separately held liquid samples; and a controller coupled with thelight sensor, the controller configured to analyze the detectedcharacteristic of the light sensed from each of the separately heldliquid samples to detect occurrences of the characteristic change inlight from respective ones of the separately held liquid samples as theseparately held liquid samples are incubated, the controller furnishingan indication that presence of the target microbe is detected while theseparately held liquid samples are being incubated when a detectednumber of occurrences of the characteristic change in light,corresponding to a number of separately held liquid samples, exceeds athreshold number of occurrences of the characteristic change in lightfrom respective ones of a total number of separately held liquidsamples, the threshold number of occurrences of the characteristicchange in light from respective ones of the total number of separatelyheld liquid samples corresponding to a concentration of the targetmicrobe within the liquid samples that indicates presence of the targetmicrobe, wherein the concentration of the target microbe within theseparately held liquid samples is indicated by the occurrence of thecharacteristic change in light from a predetermined number of theseparately held liquid samples following a predetermined incubationperiod, and wherein the threshold number of occurrences of thecharacteristic change in light from respective ones of the total numberof separately held liquid samples is less than the predetermined numberfor an intermediate time period less than the predetermined incubationperiod, and wherein the controller furnishes an indication of theabsence of the target microbe if the detected number of occurrences ofthe characteristic change in light from respective ones of the totalnumber of separately held liquid samples does not exceed the thresholdnumber of occurrences of the characteristic change in light fromrespective ones of the total number of separately held liquid sampleswithin the intermediate time period.
 2. The system as recited in claim1, wherein the liquid samples further include a growth accelerant foraccelerating growth of the target microbe.
 3. The system as recited inclaim 1, wherein the target microbe comprises Escherichia coli (E.coli).
 4. The system as recited in claim 1, further comprising a dividerfor receiving a single liquid sample and equally portioning the singleliquid sample into the liquid samples, the plurality of compartments ofthe tray configured for receiving the liquid samples from the divider.5. The system as recited in claim 1, further comprising a light sourceconfigured to be received in the incubator for illuminating the liquidsamples held in the plurality of compartments during incubation of theliquid samples.
 6. The system as recited in claim 5, wherein the lightsource is configured to individually illuminate one of the liquidsamples held in the plurality of compartments at a time.
 7. A system fordetecting the presence of a target microbe comprising: a tray configuredto be received in an incubator for holding liquid samples duringincubation, each of the liquid samples being separately held in the trayand including an indicator configured to produce a characteristic changein light from a respective one of the separately held liquid sampleswhen a target microbe metabolizes the indicator; a light sensor forsensing light from the separately held liquid samples tray duringincubation of the liquid samples, the light sensor configured to detecta characteristic of the light sensed; and a controller coupled with thelight sensor, the controller configured to analyze the detectedcharacteristic of the light sensed from the separately held liquidsamples to detect occurrences of the characteristic change in light fromrespective ones of the separately held liquid samples, the controllerfurnishing an indication that presence of the target microbe is detectedwhile the separately held liquid samples are being incubated when adetected number of occurrences of the characteristic change,corresponding to a number of separately held liquid samples, exceeds athreshold number of occurrences of the characteristic change in lightfrom respective ones of a total number of separately held liquidsamples, the threshold number of occurrences of the characteristicchange in light from respective ones of the total number of separatelyheld liquid samples corresponding to a concentration of the targetmicrobe within the liquid samples that indicates presence of the targetmicrobe, wherein the concentration of the target microbe within theseparately held liquid samples is indicated by the occurrence of thecharacteristic change in light from a predetermined number of theseparately held liquid samples following a predetermined incubationperiod, and wherein the threshold number of occurrences of thecharacteristic change in light from respective ones of the total numberof separately held liquid samples is less than the predetermined numberfor an intermediate time period less than the predetermined incubationperiod, and wherein the controller furnishes an indication of theabsence of the target microbe if the detected number of occurrences ofthe characteristic change in light from respective ones of the totalnumber of separately held liquid samples does not exceed the thresholdnumber of occurrences of the characteristic change in light fromrespective ones of the total number of separately held liquid sampleswithin the intermediate time period.
 8. The system as recited in claim7, wherein the liquid samples further include a growth accelerant foraccelerating growth of the target microbe.